Anaphylaxis is an acutesystemic (whole body) type of allergic reaction. It occurs when a person has become sensitized to a certain substance or allergen (that is, the immune system has been abnormally triggered to recognize that allergen as a threat to the body).

On the second or subsequent exposure to the substance, an allergic reaction occurs. This reaction is sudden, severe, and involves the whole body.

Tissues in different parts of the body release histamine and other substances. This causes constriction of the airways, resulting in wheezing; difficulty breathing; and gastrointestinal symptoms such as abdominal pain, cramps, vomiting, and diarrhea.

Histamine causes the blood vessels to dilate (which lowers blood pressure) and fluid to leak from the bloodstream into the tissues (which lowers the blood volume), resulting in shock. Fluid can leak into the alveoli (air sacs) of the lungs, causing pulmonary edema.

Hives and angioedema (hives on the lips, eyelids, throat, and/or tongue) often occur, and angioedema may be severe enough to cause obstruction of the airway. Prolonged anaphylaxis can cause heart arrhythmias.

Some drugs (polymyxin, morphine, X-ray dye, and others) may cause an anaphylactoid reaction (anaphylactic-like reaction) on the first exposure. This is usually from a toxic or idiosyncratic reaction rather than the "immune system" mechanism that occurs with "true" anaphylaxis, though the symptoms, risk for complications without treatment, and therapy are the same.

Anaphylaxis can occur in response to any allergen. Common causes include insect bites/stings, horse serum (used in some vaccines), food allergies, and drug allergies. Pollens and other inhaled allergens rarely cause anaphylaxis. Some people have an anaphylactic reaction with no identifiable cause.

Anaphylaxis occurs infrequently. However, it is life-threatening and can occur at any time. Risks include prior history of any type of allergic reaction.

Symptoms may include the following:

• Difficulty breathing
• Wheezing
• Abnormal (high-pitched) breathing sounds
• Confusion
• Slurred speech
• Rapid or weak pulse
• Blueness of the skin (cyanosis), including the lips or nail beds
• Fainting, light-headedness, dizziness
• Hives and generalized itching
• Anxiety
• Sensation of feeling the heart beat (palpitations)
• Nausea, vomiting
• Diarrhea
• Abdominal pain or cramping
• Skin redness
• Nasal congestion
• Cough

Note: Symptoms develop rapidly, often within seconds or minutes.

Examination of the skin may show hives and angioedema (swelling of the eyes or face). The skin may be blue (cyanosis) from lack of oxygen or may be pale from shock. Angioedema in the throat may be severe enough to block the airway.

Listening to the lungs with a stethoscope (auscultation) may reveal wheezing or indicate fluid (pulmonary edema). The pulse is rapid, and blood pressure may be low. Weakness, pale skin, heart arrhythmias, mental confusion, and other signs may indicate shock.

Testing for the specific allergen that caused anaphylaxis (if the cause is not obvious) is postponed until after treatment.

Anaphylaxis is an emergency condition requiring immediate professional medical attention. Assessment of the ABC's (airway, breathing, and circulation from Basic Life Support) should be done in all suspected anaphylactic reactions.

CPR should be initiated if indicated. People with known severe allergic reactions may carry an Epi-Pen or other allergy kit, and should be assisted if necessary. Emergency interventions by paramedics or physicians may include placing a tube through the nose or mouth into the airway (endotracheal intubation) or emergency surgery to place a tube directly into the trachea (tracheostomy or cricothyrotomy).

Epinephrine should be given by injection without delay. This opens the airways and raises the blood pressure by constricting blood vessels.

Treatment for shock includes intravenous fluids and medications that support the actions of the heart and circulatory system.

Antihistamines, such as diphenhydramine; and corticosteroids, such as prednisone may be given to further reduce symptoms (after lifesaving measures and epinephrine are administered).

Anaphylaxis is a severe disorder which has a poor prognosis without prompt treatment. Symptoms, however, usually resolve with appropriate therapy, underscoring the importance of action.

• Shock
• Cardiac arrest (no effective heartbeat)
• Respiratory arrest (absence of breathing)
• Airway obstruction

Go to the emergency room or call the local emergency number (such as 911) if severe symptoms of anaphylaxis develop.

Avoid known allergens. Any person experiencing an allergic reaction should be monitored, although monitoring may be done at home in mild cases.

Occasionally, people who have a history of drug allergies may safely be given the offending medication after pretreatment with corticosteroids (prednisone) and antihistamines (diphenhydramine).

People who have a history of allergy to insect bites/stings should be instructed to carry (and use) an emergency kit consisting of injectable epinephrine and chewable antihistamine. They should also wear a Medic-Alert or similar bracelet/necklace stating their allergy.